fix: close review findings on the velocity-curve deck and bipolar curves

Cancels the curve-node drag whenever the popup closes so Esc mid-drag can't alias the amp curve; generalizes CurveTarget routing to one switch; fixes stale/overstated comments; clamps a pre-v12 depth fold; adds deck-inertness and filter-fold test coverage.

src/shell/instrument/editor_input_curve.cpp

@@ -23,7 +23,7 @@ bool ReaSamplerEditor::handlePopupMouseDown(int w, int h, int x, int y) {
     if (curvePopup_ == CurveTarget::kNone) return false;
     const CurvePopupLayout pl = computeCurvePopup(w, h);
     if (contains(pl.close, x, y)) {
-        curvePopup_ = CurveTarget::kNone;
+        closeCurvePopup();
         invalidate();
         return true;
     }
@@ -32,7 +32,7 @@ bool ReaSamplerEditor::handlePopupMouseDown(int w, int h, int x, int y) {
         return true;
     }
     if (popupOutsideSheet(pl, x, y) && drag_ == DragKind::kNone) {
-        curvePopup_ = CurveTarget::kNone;
+        closeCurvePopup();
         invalidate();
     }
     return true;